How galaxies lose their angular momentum

The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disk galaxies that are too compact with respect to the observations. High-resolution N-body/SPH simulations in a cosmological context are presented including cold gas and dark matter. A halo with quiet merging activity since z~3.8 and with a high spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disk. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction plays a minor role,in contrast to previous claims. In fact, after this event a new extended disk begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disk galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered star burst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected as a result of these processes, late-type galaxies could form with a dominant extended disk component, resulting from late infall, a small bulge-to-disk ratio and a low baryon fraction, in agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution figures to the author

The baryon fraction in hydrodynamical simulations of galaxy clusters

We study the baryon mass fraction in a set of hydrodynamical simulations of galaxy clusters performed using the Tree+SPH code GADGET-2. We investigate the dependence of the baryon fraction upon the radiative cooling, star formation, feedback through galactic winds, conduction and redshift. Both the cold stellar component and the hot X-ray emitting gas have narrow distributions that, at large cluster-centric distances r>R500, are nearly independent of the physics included in the simulations. Only the non-radiative runs reproduce the gas fraction inferred from observations of the inner regions (r ~ R2500) of massive clusters. When cooling is turned on, the excess star formation is mitigated by the action of galactic winds, but yet not by the amount required by observational data. The baryon fraction within a fixed overdensity increases slightly with redshift, independent of the physical processes involved in the accumulation of baryons in the cluster potential well. In runs with cooling and feedback, the increase in baryons is associated with a larger stellar mass fraction that arises at high redshift as a consequence of more efficient gas cooling. For the same reason, the gas fraction appears less concentrated at higher redshift. We discuss the possible cosmological implications of our results and find that two assumptions generally adopted, (1) mean value of Yb = fb / (Omega_b/Omega_m) not evolving with redshift, and (2) a fixed ratio between f_star and f_gas independent of radius and redshift, might not be valid. In the estimate of the cosmic matter density parameter, this implies some systematic effects of the order of Delta Omega_m/Omega_m < +0.15 for non-radiative runs and Delta Omega_m/Omega_m ~ +0.05 and < -0.05 for radiative simulations.Comment: 10 pages, to appear in MNRA

MASS SEGREGATION IN DARK MATTER MODELS.

We use the moments of counts of neighbors as given by the Generalized Correlation Integrals, to study the clustering properties of Dark Matter Halos (DH) in Cold Dark Matter (CDM) and Cold+Hot Dark Matter (CHDM) models. We compare the results with those found in the CfA and SSRS galaxy catalogs. We show that if we apply the analysis in redshift space, both models reproduce equally well the observed clustering of galaxies. Mass segregation is also found in the models: more massive DHs are more clustered compared with less massive ones. In redshift space, this mass segregation is reduced by a factor 2-3 due to the peculiar velocities. Observational catalogs give an indication of luminosity and size segregation, which is consistent with the predictions of the models. Because the mass segregation is smaller in redshift space, it is suggestive that the real luminosity or size segregation of galaxies could be significantly larger than what it is found in redshift catalogs.Comment: 13 pages including 9 figures (220 KB) in uuencoded compressed Postscript format. To appear in The Astrophysical Journal, June 10. Latex file and figures available at ftp://astrohp.ft.uam.es/pub/preprints/masse

How does gas cool in DM halos?

In order to study the process of cooling in dark-matter (DM) halos and assess how well simple models can represent it, we run a set of radiative SPH hydrodynamical simulations of isolated halos, with gas sitting initially in hydrostatic equilibrium within Navarro-Frenk-White (NFW) potential wells. [...] After having assessed the numerical stability of the simulations, we compare the resulting evolution of the cooled mass with the predictions of the classical cooling model of White & Frenk and of the cooling model proposed in the MORGANA code of galaxy formation. We find that the classical model predicts fractions of cooled mass which, after about two central cooling times, are about one order of magnitude smaller than those found in simulations. Although this difference decreases with time, after 8 central cooling times, when simulations are stopped, the difference still amounts to a factor of 2-3. We ascribe this difference to the lack of validity of the assumption that a mass shell takes one cooling time, as computed on the initial conditions, to cool to very low temperature. [...] The MORGANA model [...] better agrees with the cooled mass fraction found in the simulations, especially at early times, when the density profile of the cooling gas is shallow. With the addition of the simple assumption that the increase of the radius of the cooling region is counteracted by a shrinking at the sound speed, the MORGANA model is also able to reproduce for all simulations the evolution of the cooled mass fraction to within 20-50 per cent, thereby providing a substantial improvement with respect to the classical model. Finally, we provide a very simple fitting function which accurately reproduces the cooling flow for the first ~10 central cooling times. [Abridged]Comment: 15 pages, accepted by MNRA

Simulating the formation of a proto-cluster at z~2

We present results from two high-resolution hydrodynamical simulations of proto-cluster regions at z~2.1. The simulations have been compared to observational results for the socalled Spiderweb galaxy system, the core of a putative proto-cluster region at z = 2.16, found around a radio galaxy. The simulated regions have been chosen so as to form a poor cluster with M200~10^14 h-1 Msun (C1) and a rich cluster with M200~2x10^15 h-1 Msun (C2) at z = 0. The simulated proto-clusters show evidence of ongoing assembly of a dominating central galaxy. The stellar mass of the brightest cluster galaxy (BCG) of the C2 system is in excess with respect to observational estimates for the Spiderweb galaxy, with a total star formation rate which is also larger than indicated by observations. We find that the projected velocities of galaxies in the C2 cluster are consistent with observations, while those measured for the poorer cluster C1 are too low compared to the observed velocities. We argue that the Spiderweb complex resemble the high-redshift progenitor of a rich galaxy cluster. Our results indicate that the included supernovae feedback is not enough to suppress star formation in these systems, supporting the need of introducing AGN feedback. According to our simulations, a diffuse atmosphere of hot gas in hydrostatic equilibrium should already be present at this redshift, and enriched at a level comparable to that of nearby galaxy clusters. The presence of this gas should be detectable with future deep X-ray observations.Comment: 6 pages, 4 figures, accepted for publication in MNRAS (Letters

Simulated X-ray galaxy clusters at the virial radius: slopes of the gas density, temperature and surface brightness profiles

Using a set of hydrodynamical simulations of 9 galaxy clusters with masses in the range 1.5 10^{14} M_sun < M_vir < 3.4 10^{15} M_sun, we have studied the density, temperature and X-ray surface brightness profiles of the intracluster medium in the regions around the virial radius. We have analyzed the profiles in the radial range well above the cluster core, the physics of which are still unclear and matter of tension between simulated and observed properties, and up to the virial radius and beyond, where present observations are unable to provide any constraints. We have modeled the radial profiles between 0.3 R_200 and 3 R_200 with power laws with one index, two indexes and a rolling index. The simulated temperature and [0.5-2] keV surface brightness profiles well reproduce the observed behaviours outside the core. The shape of all these profiles in the radial range considered depends mainly on the activity of the gravitational collapse, with no significant difference among models including extraphysics. The profiles steepen in the outskirts, with the slope of the power-law fit that changes from -2.5 to -3.4 in the gas density, from -0.5 to -1.8 in the gas temperature, and from -3.5 to -5.0 in the X-ray soft surface brightness. We predict that the gas density, temperature and [0.5-2] keV surface brightness values at R_200 are, on average, 0.05, 0.60, 0.008 times the measured values at 0.3 R_200. At 2 R_200, these values decrease by an order of magnitude in the gas density and surface brightness, by a factor of 2 in the temperature, putting stringent limits on the detectable properties of the intracluster-medium (ICM) in the virial regions.Comment: 13 pages, 6 figures; added reference and other minor change

Evolution of stellar-gaseous disks in cosmological haloes

We explore the growth and the evolution of the bar instability in stellar-gaseous disks embedded in a suitable dark matter halo evolving in a fully consistent cosmological framework. The aim of this paper is to point out the impact of different gas fractions on the bar formation, inside disks of different disk-to-halo mass ratio, and the role of the cosmological framework. We perform cosmological simulations with the same disk-to-halo mass ratios as in a previous work where the gas was not taken into account. We compare results of the new simulations with the previous ones to investigate the effect of the gas by analysing the morphology of the stellar and gaseous components, the stellar bar strength and the behaviour of its pattern speed. In our cosmological simulations, inside dark-matter dominated disks, a stellar bar, lasting 10 Gyr, is still living at z=0 even if the gaseous fraction exceeds half of the disk mass. However, in the most massive disks we find a threshold value (0.2) of the gas fraction able to destroy the bar. The stellar bar strength is enhanced by the gas and in the more massive disks higher gas fractions increase the bar pattern speed